1. Field of the Invention
The invention relates to a method for the preparation of sorbents for treatment of industrial effluent gases, and, more specifically, this invention relates to a method for the preparation of amine and glycol-based sorbents for the removal of carbon dioxide (CO2) from the atmosphere or CO2 generated from other sources such as power plants.
2. Background of the Invention
Fossil fuels supply more than 98% of the world's energy requirements. The combustion of fossil fuels, however, is one of the major sources of the greenhouse gas, CO2. The ability to efficiently and safely absorb CO2 is important in the development and application of cost-effective technologies for CO2 removal from gas streams.
Separation and capture processes of CO2, as those processes relate to ocean and/or geologic sequestration, have been identified as a high-priority research topic in Department of Energy (DOE) reports. The costs of separation and capture, including compression to the required CO2 pressure for the sequestration step, are generally estimated to comprise about three-fourths of the total cost of ocean or geologic sequestration. An improvement of the separation and capture of CO2 will reduce the total cost required for sequestration.
CO2 absorption processes using aqueous amine solutions facilitate the removal of CO2 from gas streams in some industries. These processes often are referred to as wet chemical stripping.
Wet chemical stripping of CO2 involves one or more reversible chemical reactions between CO2 and amine substances to produce a liquid species, such as a carbamate. Upon heating, the carbamate breaks down to free CO2 with the original amine regenerated to subsequently react with additional CO2. An example of the process is given by Equation 1: 
The two moieties represented by R may be either any alkyl moiety, any aryl moiety, or any combination thereof. At high CO2 concentrations, bicarbonate formation may also take place. Where R is an ethanolic moiety (i.e., in the presence of the reactant monoethanolamine (MEA) as shown infra), the reaction proceeds as follows: 
Typically, these amines, MEA and DEA, are used as 25 to 30 wt. % amine in aqueous solution. The amine solution enters the top of an absorption tower while the carbon dioxide containing gaseous stream is introduced at the bottom. During contact with the CO2-containing gaseous stream, the amine solution chemically absorbs the CO2 from the gaseous stream to create a carbamate. Conversion of carbamate ion back to CO2 proceeds through a thermal regeneration process, typically at a temperature of about 120° C. Carbon dioxide and water emerge from the amine solution and the water is separated via condensation using a heat exchanger. After regeneration, the amine solution is recycled back to the absorption tower for additional CO2 absorption.
Carbon dioxide capture and regeneration in the above-described manner requires high temperatures. As such, the process outlined supra is energy intensive. Further, the amine solution has a limited lifetime due to degradation through oxidation of the amine. In addition, high amine concentrations and high CO2 loadings exacerbate corrosion problems of process equipment.
Solid sorbents serve as alternatives to wet chemical stripping via the formation of carbamate species. However, solid sorbents' absorption capabilities are limited by their respective surface areas inasmuch as only the sorbent's surface is treated with CO2-reactive compounds such as an amine or an ether. Since only the surface is involved in the reaction, a very limited quantity of CO2-reactive material can be incorporated in the sorbent solid by either surface modification or solid impregnation techniques. This severely restricts the amount of gases such as CO2 that can be absorbed by the sorbents currently present in the art and gives rise to short breakthrough times. The breakthrough time is that amount of time by the end of which the sorbent is saturated with the gas being absorbed, after which that gas's concentration level in effluent gas begins to rise.
Several solid sorbents have recently been utilized to remove CO2 from enclosed environments. Important considerations include the ability to regenerate an absorbent and the ease of its regeneration. Efforts have been made to reversibly adsorb CO2 on a silica gel first modified with amine. O. Leal, et al., Inorganica Chimica Acta, 240, 183-189, 1995. Surface modification occurs when the hydroxyl moieties of the silica gel surface bonds with chemical moieties. When the chemical moiety is 3-aminopropyltriethoxysilane, bonding occurs between the oxygen atoms of the ethoxy moieties and silicon atoms at the surface of the gel. It is this surface modification that facilitates adsorption of CO2 via the formation of carbamate species.
U.S. Pat. Nos. 5,876,488 and 5,492,683 both awarded to Birbara, et al. on Mar. 2, 1999, and Feb. 20, 1996, respectively, disclose a method for incorporating liquid amines onto the surface of a support substrate that has a surface area greater than 50 square meters per gram (m2/g). The reaction is restricted to the surface and the method requires materials that have high surface areas.
U.S. Pat. No. 5,087,597 awarded to Leal, et al. on Feb. 11, 1992 discloses a method for the chemisorption of CO2 at room temperature using a silica gel having a surface area of between 120 and 240 m2/g. The gel has been modified with a polyalcoxisilane containing one or more amino moieties in its structure.
U.S. Pat. No. 4,810,266 awarded to Zinnen et al. on Mar. 7, 1989 discloses a method for CO2 removal using animated carbon molecular sieves that have been treated with alcohol amines.
None of the aforementioned patents discloses a sorbent with an absorption capacity independent of the sorbent's surface area nor any method or process for fabricating such a sorbent.
A need exists in the art for a process to produce sorbents with wide capabilities in cold-gas cleanup. In addition, there is a need for a sorbent which incorporates a much higher weight percentage of the CO2-reactive moiety into the sorbent support phase thus giving rise to a greater absorption capability and breakthrough time for CO2. Further, incorporation of the reactive moiety into the support phase will avoid the corrosion and evaporation problems inherent with use of state of the art reactive organic liquids/solids, now present in the art, such as corrosion, and evaporation. In particular, there is a need for sorbents whose absorption capabilities are independent of their surface areas. In addition, the sorbent should be easily regenerated at ambient to moderate temperatures for use in additional absorption/desorption cycles. Finally, the materials used in sorbent preparation should be inexpensive.